My Puerto Rican Great Grandmother's first name was Dionicia spelled with a c instead of an s. Perhaps it is the Spanish version of Dionisia. She was born in the late 1800's.
Dionisia is indeed the medieval English form of Denise. However, today Dionisia is used in Spanish and occasionally Italian. It's Portuguese too, but it's written as Dionísia. It's these countries' equivalent of Denise.